Recently, a demand for high recording density has been increased on a magnetic disk to be mounted on information processing devices such as a hard disk drive. Under such a situation, a glass substrate excellent in flatness and substrate strength has been widely used in place of conventional aluminum substrates.
For example, the glass substrate for magnetic disk is produced by making a circular hole in the center of a circular glass plate, successively performing chamfering, main surface lapping, and end surface mirror polishing, and polishing the main surface of the circular glass plate with a polishing pad.
In order to increase recording capacity of the magnetic disk, it is necessary to enlarge a recording area, so that the main surface of the glass substrate for magnetic disk is desirably flat to more outer peripheral side. FIG. 1 is a cross-sectional view schematically illustrating the vicinity of end surface of the glass substrate for magnetic disk after polishing of the main surface. In the FIGURE, a is a chamfer surface (chamfered surface), b is an outer peripheral end surface, c is a main surface outer peripheral portion, d is a boundary between the chamfer surface a and the main surface outer peripheral portion c. Roll-off (end surface sagging) is continuously formed from the main surface outer peripheral portion c to the chamfer surface and hence the recording area is reduced.
Moreover, the straight line shown by a dotted line in FIG. 1 is a base line g for determining a degree of the roll-off. The base line g is determined as a straight line which overlaps with a portion f of the main surface outer peripheral portion c 2.5 mm or more and 5 mm or less apart from the boundary d toward the center of the main surface or as a straight line which comes nearest to the portion f. Of the main surface outer peripheral portion c, the portion 0.25 mm or more and 5 mm or less apart from the boundary d toward the center of the main surface is a roll-off measuring region e. The degree of the roll-off is a difference between the largest height and the smallest height of the main surface outer peripheral portion c from the base line g in the roll-off measuring region e.
Thus, in order to increase the recording capacity, it is essential to reduce the roll-off and hitherto, an improvement of the polishing pad has been performed. For example, polishing the glass plate with a polishing pad having an increased Asker-A hardness of 90 or more by containing a predetermined amount of an abrasive has been proposed (see, Patent Document 1).
Patent Document 1: JP-A-2007-250166